The invention relates to tris(oxalato)phosphates, M[P(C2O4)3], a method for their preparation and the use of tris(oxalato)phosphates, inter alia, as conducting salts in electrochemical storage systems.
Electrochemical storage systems are e.g. batteries and so-called supercapacitors. Electrolyte solutions comprising a conducting salt and an aprotic solvent are used in these systems. Modern systems, such as e.g. lithium ion batteries, have a high power density and output voltage (oftenxe2x89xa73 V). Aprotic electrolyte systems are required for these cells.
Lithium hexafluorophosphate (LiPF6) is currently used as the conducting salt in all commercial lithium ion batteries. This salt has the necessary prerequisites for use in high-energy cells, i.e. it is readily soluble in aprotic solvents, it leads to electrolytes with high conductivities, and it has a high degree of electrochemical stability. Oxidative decomposition occurs only at potentials greater than approx 4.5 V.
However, LiPF6 also has serious disadvantages which are chiefly attributed to its lack of thermal stability. A dissociation, although slight, into LiF and PF, which can lead to cationic polymerization of the solvent caused by the Lewis acid PF., takes place in solution.
On contact with moisture, corrosive hydrogen fluoride is liberated, which on the one hand makes handling difficult because of its toxicity and corrosiveness, and on the other hand can lead to (partial) dissolution of the transition metal oxides employed as the cathode material (e.g. LiMn2O4). The cycle stability of the electrochemical energy store concerned is affected in this manner.
Against this background, there have been intensive efforts with the aim of developing alternative conductive salts. Above all, lithium salts with perfluorinated organic radicals have been tested as such salts. In particular lithium trifluoromethanesulfonate, lithium bis(trifluoromethanesulfonyl)-imide and the lithium methides, the simplest parent substance of which is lithium tris(trifluoromethanesulfonyl)methide, have been mentioned. These salts also have disadvantages which have so far impeded their use in commercial lithium batteries. The first mentioned salt does not impart a sufficiently high conductivity to the electrolytes prepared with it. The salts mentioned last indeed have a conductivity equivalent to that of LiPF6, but they are of no commercial interest because of the expensive preparation process. Furthermore, the imide is corrosive towards aluminium sheets, which are employed as current diverters in many battery systems. Because of the high fluorine content of the compounds, exothermic reactions with the lithium of the electrode are, moreover, to be feared under adverse conditions.
Lithium hexafluorophosphate and all the abovementioned conductive salt alternatives have the common feature of a rather high fluorine content. On the basis of this fact, the preparation costs are comparatively high and certain safety precautions have to be taken in the disposal or recycling of spent batteries in order to avoid emission of fluorine-containing substances (e.g. toxic and corrosive hydrogen fluoride HF).
The lithium borate complex salts [(Rxe2x80x2O)2B(ORxe2x80x3)2]Li described in DE 19633027 A1 represent a considerable advance. In these, Rxe2x80x2 and Rxe2x80x3 are the same or different, Rxe2x80x2 and Rxe2x80x3 are optionally bonded to one another by a single or double bond, and Rxe2x80x2 and Rxe2x80x3 in each case individually or together denote an aromatic ring from the group consisting of phenyl, naphthyl, anthracenyl or phenanthrenyl, which can be unsubstituted or mono- to tetrasubstituted by A or Hal, wherein Hal represents fluorine or chlorine and A is an alkyl residue having 1 to 8 C atoms, which can in turn be mono- to tetrahalogenated.
The stabilities of the non-fluorinated derivatives, which are indeed improved, but are in no way adequate for the 3 V systems required, are a disadvantage of these compounds. Thus, e.g. unsubstituted lithium bis[1,2-benzenediolato(2-)-O,Oxe2x80x2]borate(1-) (which is the 2:1 complex of pyrocatechol) already decomposes when an anodic potential of 3.6 V is exceeded. This value is significantly below that of the standard conductive salt LiPF6 (approx. 4.5 V). Even in the case of these chelatoborates, too, only fluorine-substituted derivatives are sufficiently stable to oxidation.
A chelatophosphate, namely lithium tris[1,2-benzenediolato(2)-O,Oxe2x80x2]phosphate, has been tested as another alternative (M. Handa, M. Suzuki, J. Suzuki, H. Kanematsu, Y. Sasaki, Electrochemical and Solid-State Letters, 2 (2) 60-62 (1999)). This salt has a wider electrochemical stability range than the corresponding boron compound (start of decomposition from about 3.7 V), but the maximum conductivities which can be achieved for electrolyte solutions prepared with this are below 4 mS/cm; i.e. significantly below the standard given by LiPF6.
Salts with large cations (e.g. N(R1R2R3R4)+, wherein R1, R2, R3 and R4 are independently of one another H or an alkyl group having 1 to 8 C atoms, are commonly used for supercapacitors since these are largely inert towards the electrode materials.
The invention is therefore based on the object of eliminating the disadvantages of the prior art and of providing halogen-free electrochemically stable and thermally stable compounds which are readily soluble in aprotic solvents and are suitable for the preparation of electrolyte solutions with a good conductivity. Furthermore, catalysts should be found, such as those employed, for example, for the hydroamination of amines. The invention is furthermore based on the object of providing a method for the preparation of these compounds.
The object is achieved with tris(oxalato)phosphates of the general formula M[P(C2O4)3], where N=H, a metal or N(R1R2R3R4), wherein R1, R2, R3 and R4 independently of one another are H or an alkyl group having 1 to 8 C atoms. Metal tris (oxalato) phosphates are as preferred compounds and hydrogen tris(oxalato)phosphate, lithium tris (oxalato) phosphate and sodium tris(oxalato)phosphate are as particularly preferred compounds.
It has been found, surprisingly, that these tris(oxalato)phosphates have the required property profiles and furthermore are easy to prepare. The compounds are readily to very readily soluble in polar-aprotic solvents.